The Sensory System

Question 1

Define sensory perception

Answer 1

The neurophysiological processing of stimuli in their environment.

Question 2

How are sensory receptors classified? Give examples for each.

Answer 2

Structure -
Free nerve endings - nociceptors, thermoreceptors
Encapsulated - muscle spindle, pacinian corpuscles, merkle’s disc and meissner’s corpuscle.
Seperate cells - taste buds (chemoreceptors)

Location -
Interoreceptors - found in glands/organs
Exteroreceptors - found on the skin
Proprioceptors - joints/muscles

Stimulus -
Pain, temperature, auditory, visual, pressure, chemical, taste.

Question 3

What is the receptor that detects pressure? How does this occur?

Answer 3

Pacinian corpuscle.
The sensory nerve ending is encapsulated by many layers called lamellae. If you press on these they will deform and press on each other, eventually pressing on the nerve tip. This makes it leaky to sodium ion channels, causing an action potential.

Question 4

Describe/draw the parts to a muscle spindle. What sensory information does it detect/relay?

Answer 4

Extrafusal fibres - the contractile parts - NOT part of the muscle spindle.
Intrafusal fibres - 2 types, nuclear bag and nuclear chain fibres that are encapsulated
Sensory neurones and their endings
Motor neurones

Question 5

What is the role of the alpha motor neurone?

Answer 5

Causes the extrafusal fibres to contract - responsible for voluntary movement of skeletal muscle fibres.

Question 6

What is the role of the gamma motor neurone?

Answer 6

To ‘reset’ the intrafusal fibres and set their sensitivity to stretch so that they become taut again after being stretched so that they can continue to send sensory information.

Question 7

Name the 2 sensory neurones and their endings.

Answer 7

Primary annulospiral endings - twist round the intrafusal fibres
Secondary flower spray endings

Question 8

Define proprioception and give 3 functional examples of proprioception

Answer 8

The ability to sense the position of your limbs in space.
Putting a key in a key hole in the dark
Touch typing/playing the piano whilst looking at music.
Driving the car and changing gear whilst looking at the road.
Complex movements e.g. with the cerebellum acting as comparator.
Tying your shoe laces in absence of vision

Question 9

Give 2 functional examples of light touch

Answer 9

Turning paper in a magazine.

Feeling your clothes on your skin.

Question 10

Give 2 functional examples of pressure

Answer 10

Shaking hands with someone

Picking up a cup/your phone

Question 11

Give 2 functional examples of temperature

Answer 11

Touching a warm radiator

Cold rain and wind on your face

Question 12

Give 2 functional examples pain

Answer 12

Touching a hot plate

Cutting yourself

Question 13

Define stereognosis. Give a functional example

Answer 13

The ability to detect what an object is without visual stimuli.
Braille.

Question 14

Describe the differences between rapidly adaptive (phasic) and slowly adaptive receptors

Answer 14

Rapidly adaptive receptors - detects a change in the environment and sends feedback to the brain e.g. you put your clothes on. These will then ‘switch off’ until there is another change in your environment, e.g. you move/take a layer off.

Slowly adaptive receptors - continuous feedback of the position of your joints and stretch receptors from muscles/tendons to help make postural adjustments to help with stability, balance and efficiency of movements.

Question 15

What is 2 point discrimination? What is it affected by? Which receptors?

Answer 15

The ability to detect that 2 nearby objects are in fact 2 separate objects, not 1. The size, density, overlap of receptive fields. Receptors are the merkle’s disc and meissners corpuscles (light touch)

Question 16

Define receptive field

Answer 16

An area around a sensory neurone that if stimulated will cause a response by a sensory neurone.

Question 17

What is the role of the primary sensory cortex?

Answer 17

Detects and perceives sensory stimuli and discriminates between the quality and intensity of it.

Question 18

What is the role of the sensory association area?

Answer 18

Discriminates more complex sensory stimuli e.g. stereognosis. It uses previous experiences of stored sensory information compare/analyse stimuli.

Question 19

Which pathways relay information involving pain/temperature, tickle and itch? Classify these receptors.

Answer 19

Pain and temperature (free nerve endings, noci and thermoreceptors, exteroreceptors - lateral spinothalamic tracts

Tickle and itch - anterior spinothalamic tracts.

Question 20

Describe the stages involved in the spinothalamic pathway

Answer 20

A harmful stimulus is detected by nociceptors on the finger e.g. cut yourself with a knife. Local depolarisation and a graded potential results with an action potential being generated. The first order neurone relays this sensory information to the dorsal horn of the spinal cord via the dorsal root ganglion. At the spinal cord it synapses immediately with the second order neurone.

The second order neurone crosses over to the other side of the spinal cord and ascends the spinal cord via the lateral spinothalamic tract towards the brainstem, then to the thalamus. The 3rd order neurone then relays the sensory information to the primary sensory cortex in the post-central gyrus of the parietal lobe.

Question 21

What sensory information does the posterior column medial lemniscus pathway relay? What are the receptors for each? Classify these receptors.

Answer 21

Proprioception - muscle spindle and stretch of the muscle (mechanical forces)
Light touch - Meissners corpuscle, merkles disc (encapsulated)
Pressure - pacinian corpuscle - encapsulated

Question 22

Describe the stages involved in the posterior column medial lemniscus pathway.

Answer 22

A stimulus e.g. movement in the ankle causes the muscle to stretch which is detected by the muscle spindle. This leads to local depolarisation, a graded potential and an action potential. The sensory afferent neurones (first order neurone) relay the somatosensation via the dorsal root ganglion to the dorsal horn of the spinal cord. This continues up the gracile tract (cuneate if upper limb) to the gracile nucleus in the medulla where it synapses. Here the second order neurone crosses over at the medulla as internal arcuate fibres to form the medial lemniscus.

The sensory information continues up the brain stem to the ventral posterolateral nucleus in the thalamus. Information is carried to the posterior limb of the internal capsule. Here the second order neurone synapses with the third order neurone which relays information from the posterior limb of the internal capsule to the primary sensory cortex in the parietal lobe, posterior to the central sulcus in the post central gyrus.